Influence of Si Addition to the Coating Bath on the Growth of Al-Fe Alloy Layers in Hot-dip Zn-Al-Mg Alloy-coated Steel Sheets

The influence of Si addition to the coating bath on the growth of Al-Fe alloy layers formed in the interface between the coating and the steel substrate was studied utilizing a Zn-Al-Mg coating bath with 10-11 mass% Al content. While the Al-Fe intermetallic compound rapidly grew in the interface between the coating and the steel substrate when dipped into a coating bath of Zn-10mass%Al-3mass%Mg at 550 degrees C, the growth was extremely suppressed by the addition of 0.2 mass% Si to the coating bath. TEM observation revealed that, in the interface between the coating and the substrate the Si addition resulted in the uniform formation of very fine grains with a size of 20-30 nm in diameter of the Fe2Al5 phase with Si and Zn in a solid solution. The mechanism of the Si addition was postulated that the large amount of Fe in the substrate may be dissolved easily into the liquid phase in the case of a ternary Fe-Al-Zn system, whereas in the case of a quaternary Fe-Al-Zn-Si system, the dissolution of Fe into the liquid phase is significantly suppressed by the presence of very fine and thin Fe2Al5 containing Si in the interface between the coating and the substrate. Thus, the growth rate of Fe2Al5 was extremely reduced.